Integrated circuits (ICs) are currently used to sense some properties of a fluid. For example, Ion-Sensitive Field Effect Transistors (ISFETs) fabricated using complementary metal-oxide semiconductor (CMOS) processing technology are used to sense an ion concentration of an electrolytic solution (i.e., ion concentrated fluid). More particularly, the ISFET is a MOSFET formed on a substrate, such as silicon, and comprises a source and drain region with a channel therebetween. The ISFET further comprises a floating gate region that is electrically connected to ion concentrated fluid via a metal sense plate and one or more other metal interconnects or structures. During operation of the ISFET, in general, the ion concentration within the fluid results in a corresponding electrical bias of the floating gate. This bias results in a measurable change in channel conductivity within the channel between the source and drain. In this manner, it can be said that the ISFET directly senses the ion concentration of the fluid through its source/drain channel conductivity.
Such an ISFET structure has some shortcomings, however. For example, the threshold voltage (Vt) at neutral pH of such devices is known to vary widely. These deleterious effects are primarily attributed to the presence of fixed charge within the floating gate dielectric (and/or a passivation layer) as well as process-induced charges within the floating gate of the ISFET. Consequently, the electrical behavior of the ISFET (with respect to a given ion concentration of the fluid) can be difficult to predict, unless sufficient care is exercised in the manufacturing phase.
Methods such as ultraviolet erase processes and voltage trimming techniques have been used to address the varying threshold voltages of ISFETs. However, in order to sense the ion concentration of the fluid, it is required that at least a portion of the IC be directly exposed to the fluid. Unless additional material cost is factored into the IC fabrication process to protect the exposed IC components, the IC can suffer corrosive effects from the fluid exposure, thereby limiting its lifetime.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.